[clang] Add tests for Core issues about friend templates #94288

Endilll · 2024-06-03T22:24:57Z

This patch covers the following Core issues:
CWG1862 "Determining “corresponding members” for friendship"
CWG1918 "friend templates with dependent scopes"
CWG1945 "Friend declarations naming members of class templates in non-templates"

All of them were resolved by CWG1862, where the current wording of [temp.friend] p5 comes from:

A template friend declaration may declare a member of a dependent type to be a friend. The friend declaration shall declare a function or specify a type with an elaborated-type-specifier, in either case with a nested-name-specifier ending with a simple-template-id, C, whose template-name names a class template. The template parameters of the template friend declaration shall be deducible from C ([temp.deduct.type]). In this case, a member of a specialization S of the class template is a friend of the class granting friendship if deduction of the template parameters of C from S succeeds, and substituting the deduced template arguments into the friend declaration produces a declaration that corresponds to the member of the specialization.

A useful example is provided after the paragraph: https://eel.is/c++draft/temp.friend#example-4.

Neither of Core issues is implemented, because we don't support dependent nested friend specifiers.

llvmbot · 2024-06-03T22:25:25Z

@llvm/pr-subscribers-clang

Author: Vlad Serebrennikov (Endilll)

Changes

This patch covers the following Core issues:
CWG1862 "Determining “corresponding members” for friendship"
CWG1918 "friend templates with dependent scopes"
CWG1945 "Friend declarations naming members of class templates in non-templates"

All of them were resolved by CWG1862, where the current wording of [[temp.friend] p5](https://eel.is/c++draft/temp.friend#5) comes from:
> A template friend declaration may declare a member of a dependent type to be a friend[.](https://eel.is/c++draft/temp.friend#5.sentence-1) The friend declaration shall declare a function or specify a type with an elaborated-type-specifier, in either case with a nested-name-specifier ending with a simple-template-id, C, whose template-name names a class template[.](https://eel.is/c++draft/temp.friend#5.sentence-2) The template parameters of the template friend declaration shall be deducible from C ([temp.deduct.type])[.](https://eel.is/c++draft/temp.friend#5.sentence-3) In this case, a member of a specialization S of the class template is a friend of the class granting friendship if deduction of the template parameters of C from S succeeds, and substituting the deduced template arguments into the friend declaration produces a declaration that corresponds to the member of the specialization[.](https://eel.is/c++draft/temp.friend#5.sentence-4)

A useful example is provided after the paragraph: https://eel.is/c++draft/temp.friend#example-4.

Neither of Core issues is implemented, because we don't support dependent nested friend specifiers.

Full diff: https://github.com/llvm/llvm-project/pull/94288.diff

3 Files Affected:

(modified) clang/test/CXX/drs/cwg18xx.cpp (+92)
(modified) clang/test/CXX/drs/cwg19xx.cpp (+39)
(modified) clang/www/cxx_dr_status.html (+3-3)

diff --git a/clang/test/CXX/drs/cwg18xx.cpp b/clang/test/CXX/drs/cwg18xx.cpp
index b71a81b62f814..054ce5a4f4b70 100644
--- a/clang/test/CXX/drs/cwg18xx.cpp
+++ b/clang/test/CXX/drs/cwg18xx.cpp
@@ -373,6 +373,98 @@ namespace cwg1837 { // cwg1837: 3.3
 #endif
 }
 
+namespace cwg1862 { // cwg1862: no
+template<class T>
+struct A {
+  struct B {
+    void e();
+  };
+  
+  void f();
+  
+  struct D {
+    void g();
+  };
+  
+  T h();
+
+  template<T U>
+  T i();
+};
+
+template<>
+struct A<int> {
+  struct B {
+    void e();
+  };
+  
+  int f();
+  
+  struct D {
+    void g();
+  };
+  
+  template<int U>
+  int i();
+};
+
+template<>
+struct A<float*> {
+  int* h();
+};
+
+class C {
+  int private_int;
+
+  template<class T>
+  friend struct A<T>::B;
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::' for friend class declaration is not supported; turning off access control for 'C'}}
+
+  template<class T>
+  friend void A<T>::f();
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::' for friend class declaration is not supported; turning off access control for 'C'}}
+
+  // FIXME: this is ill-formed, because A<T>::D does not end with a simple-template-id
+  template<class T>
+  friend void A<T>::D::g();
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::D::' for friend class declaration is not supported; turning off access control for 'C'}}
+  
+  template<class T>
+  friend int *A<T*>::h();
+  // expected-warning@-1 {{dependent nested name specifier 'A<T *>::' for friend class declaration is not supported; turning off access control for 'C'}}
+  
+  template<class T>
+  template<T U>
+  friend T A<T>::i();
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::' for friend class declaration is not supported; turning off access control for 'C'}}
+};
+
+C c;
+
+template<class T>
+void A<T>::B::e() { (void)c.private_int; }
+void A<int>::B::e() { (void)c.private_int; }
+
+template<class T>
+void A<T>::f() { (void)c.private_int; }
+int A<int>::f() { (void)c.private_int; return 0; }
+
+// FIXME: both definition of 'D::g' are not friends, so they don't have access to 'private_int' 
+template<class T>
+void A<T>::D::g() { (void)c.private_int; }
+void A<int>::D::g() { (void)c.private_int; }
+
+template<class T>
+T A<T>::h() { (void)c.private_int; }
+int* A<float*>::h() { (void)c.private_int; return 0; }
+
+template<class T>
+template<T U>
+T A<T>::i() { (void)c.private_int; }
+template<int U>
+int A<int>::i() { (void)c.private_int; }
+} // namespace cwg1862
+
 namespace cwg1872 { // cwg1872: 9
 #if __cplusplus >= 201103L
   template<typename T> struct A : T {
diff --git a/clang/test/CXX/drs/cwg19xx.cpp b/clang/test/CXX/drs/cwg19xx.cpp
index 3e4f82813f51b..6085a40b67afb 100644
--- a/clang/test/CXX/drs/cwg19xx.cpp
+++ b/clang/test/CXX/drs/cwg19xx.cpp
@@ -80,6 +80,30 @@ namespace cwg1909 { // cwg1909: 3.7
   };
 }
 
+namespace cwg1918 { // cwg1918: no
+template<typename T> struct A {
+  class B {
+    class C {};
+  };
+};
+class X {
+  static int x;
+  // FIXME: this is ill-formed, because A<T>::B::C does not end with a simple-template-id
+  template <typename T>
+  friend class A<T>::B::C;
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::B::' for friend class declaration is not supported; turning off access control for 'X'}}
+};
+template<> struct A<int> {
+  typedef struct Q B;
+};
+struct Q {
+  class C {
+    // FIXME: 'f' is not a friend, so 'X::x' is not accessible
+    int f() { return X::x; }
+  };
+};
+} // namespace cwg1918
+
 namespace cwg1940 { // cwg1940: 3.5
 #if __cplusplus >= 201103L
 static union {
@@ -121,6 +145,21 @@ derived d2(42, 9);
 #endif
 }
 
+namespace cwg1945 { // cwg1945: no
+template<typename T> struct A {
+  class B {
+    class C {};
+  };
+};
+class X {
+  static int x;
+  // FIXME: this is ill-formed, because A<T>::B::C does not end with a simple-template-id
+  template <typename T>
+  friend class A<T>::B::C;
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::B::' for friend class declaration is not supported; turning off access control for 'X'}}
+};
+} // namespace cwg1918
+
 namespace cwg1947 { // cwg1947: 3.5
 #if __cplusplus >= 201402L
 unsigned o = 0'01;  // ok
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index 4d94ac5a1ac1d..e5f79419a9975 100755
--- a/clang/www/cxx_dr_status.html
+++ b/clang/www/cxx_dr_status.html
@@ -10980,7 +10980,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1862.html">1862</a></td>
     <td>CD5</td>
     <td>Determining &#8220;corresponding members&#8221; for friendship</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="1863">
     <td><a href="https://cplusplus.github.io/CWG/issues/1863.html">1863</a></td>
@@ -11316,7 +11316,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1918.html">1918</a></td>
     <td>CD5</td>
     <td><TT>friend</TT> templates with dependent scopes</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr class="open" id="1919">
     <td><a href="https://cplusplus.github.io/CWG/issues/1919.html">1919</a></td>
@@ -11478,7 +11478,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1945.html">1945</a></td>
     <td>CD5</td>
     <td>Friend declarations naming members of class templates in non-templates</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="1946">
     <td><a href="https://cplusplus.github.io/CWG/issues/1946.html">1946</a></td>

shafik

LGTM after fixing minor comment

shafik · 2024-06-04T00:02:52Z

clang/test/CXX/drs/cwg18xx.cpp

+C c;
+
+template<class T>
+void A<T>::B::e() { (void)c.private_int; }


I guess these work b/c as the diagnostic above said "we turn off access control"?

No, A<T>::B is a friend. D::g is the only function that doesn't have access in this example. Consult https://eel.is/c++draft/temp.friend#example-4 for details.

clang/test/CXX/drs/cwg19xx.cpp

Co-authored-by: Shafik Yaghmour <shafik.yaghmour@intel.com>

[clang] Add tests for Core issues about friend templates

a813e0a

Endilll added test-suite c++ labels Jun 3, 2024

Endilll requested review from cor3ntin, AaronBallman, shafik, erichkeane, sdkrystian and Sirraide June 3, 2024 22:24

llvmbot added the clang Clang issues not falling into any other category label Jun 3, 2024

shafik reviewed Jun 4, 2024

View reviewed changes

shafik approved these changes Jun 4, 2024

View reviewed changes

Update clang/test/CXX/drs/cwg19xx.cpp

4ccff86

Co-authored-by: Shafik Yaghmour <shafik.yaghmour@intel.com>

erichkeane approved these changes Jun 4, 2024

View reviewed changes

Endilll merged commit e651ee9 into llvm:main Jun 4, 2024

Endilll deleted the cwg1862 branch June 4, 2024 13:42

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